A solar cell having a pn junction and two-sided contacting is described in DE 196 02 313 A1. The solar cell is equipped with an externally controllable field electrode to suppress the charge carrier recombination at the point of contact on the rear, which is turned away from the light. This field electrode is arranged between the contact fingers on an insulation layer on the absorber layer and is connected to an external voltage source. The field electrode at a negative potential generates an electric field in the semiconductor-insulator interface, which electric field forces the minority charge carriers diffusing into this region back towards the pn junction and, by contrast, accelerates the majority charge carriers towards rear-face contact. In order to reduce the recombination rate on the front of the solar cell, an externally controllable field electrode of opposed polarisation may also be provided there, the field effect of said field electrode forcing the majority charge carriers towards rear contact and forcing the minority charge carriers towards front contact. However, charge carrier separation takes place exclusively at the pn junction.
A solar cell contacted on the rear is described in WO 2007/022955 A1, in which the electric fields are produced for charge carrier separation in a doped absorber via alternately neighboring emitter regions (doped oppositely to the absorber) and BSF regions (highly doped identically to the absorber). However, the emitter and BSF regions are electrically conductive semiconductor regions with doping. Corresponding doping and structuring measures are necessary during production.
It is described in DE 44 12 297 A1 to passivate the recombination rate on the surface of a semiconductor element, for example p-conductive silicon, by applying a layer of an electric insulator, for example oxides or nitrides, to the surface of the semiconductor element, and then to apply electrical charges to the surface of the insulator layer. Positive or negative charges can be applied. The density of the charge carriers is thus reduced at the points of high density of states at trapping or recombination centres. An electric field is produced inside the semiconductor element by applying the charges, as a result of which electric field the freely movable minority charge carriers located in the conduction band (electrons with a p-doped semiconductor, holes in an n-doped semiconductor) are drawn from the near-surface region into the semiconductor element. The charges applied externally (for example by corona remove) may, for example, be fixed by a subsequently thermally grown cover layer. Insulation layers with charges may also be applied to both surfaces of the semiconductor element, but in this case charges of identical polarity are applied so as to measure the life span of the charge carriers.
The use of passivation layers made of aluminium oxide and silicon nitride with their different surface charges (AlO, negative, and SiN, positive) for solar cells to reduce the surface recombination rate is described in DE 35 36 299 A1. Solar cells with MIS contact over the entire surface, MIS inversion layer solar cells and solar cells with a conventional pn junction for charge carrier separation are described.
The application of Al2O3 as a passivation layer for a diffused (p+) emitter is also described in a publication by Jan Benick et al., “Surface passivation of boron diffused emitters for high efficiency solar cells” (Proceedings PVSEC-33, San Diego, May 2008). A solar cell Al2O3/cSi(p+, front emitter, planar, diffused)/c-Si(m, absorber, wafer)/SiO2 with c-Si(n+, BSF, rear-face point contacts, diffused) was processed with an efficacy of 23.2%. The structure of a processed solar cell with 20.6% efficacy (silicon nitride/cSi(n+, front emitter, planar, diffused)/c-Si(p, absorber, wafer)/Al2O3 with c-Si(p+, BSF, rear point contacts, diffused) is described in a publication by Jan Schmidt et al., “Atomic-layer-deposited aluminium oxide for the surface passivation of high efficiency silicon solar cells” (Proceedings PVSEC-33, San Diego, May 2008). In both publications there are the (diffused) (p+) and (n+) emitters and BSF regions which (via the formation of a “conventional” p+/pn or n+/p emitter contact at the emitter-absorber interface or via the formation of a p-F/p or a n+/n BSF contact at the BSF-absorber interface) enable the selective “conventional” charge carrier separation.
The publication by Jan Schmidt et al. describes a solar cell of the generic type. It is a solar cell with a photoactive, semiconductive absorber layer, in which excess positive and negative charge carriers are generated by light incident on the front of the absorber layer during operation. In order to separate the positive and negative charge carriers, an electric field is generated in the absorber layer by the formation of a pn junction between the contact, over the entire surface, of a front emitter layer and a p-doped silicon wafer (see FIG. 3 of the publication by Jan Schmidt et al.). The separated charge carriers (minority charge carriers) can move over a minimal effective diffusion length Leff,min in the absorber layer before they recombine. Further, point contacts are provided as first contact elements on the rear of the absorber layer, which is turned away from the light during operation, in order to remove the positive charge carriers. To remove the negative charge carriers, contact strips are provided as second contact elements on the front of the absorber layer, which faces the light during operation. The solar cell is thus contacted on both sides. A planar Al2O3 layer is also located on the rear of the absorber layer as an electrically insulating first passivation region with a high negative surface charge for concentration of positive charge carriers in the absorber layer in the contact region. A planar silicon nitride layer is also located on the front of the solar cell on the emitter layer as an electrically insulating second passivation region with a high positive surface charge for concentration of negative charge carriers in the absorber layer in the contact region. However, in the solar cell the charge carrier separation takes place conventionally by means of a selective pn junction between the emitter and absorber.